Process for the production of synthesis gas

ABSTRACT

A process for the production of synthesis gas for obtaining compounds such as ammonia or methanol, in which hydrocarbons and steam are reacted first in a primary reforming section ( 11 ) and then—together with oxygen—in a secondary reforming section ( 12 ), thus obtaining CO, CO 2 , H 2  and possibly N 2  which are then fed to a carbon monoxide conversion section ( 13, 14 ), is distinguished by the fact of reacting hydrocarbons, steam and oxygen in an autothermal reforming section ( 20 ) provided in parallel with respect to other reforming sections ( 11, 12 ), and feeding the so produced CO, CO 2 , H 2  and possibly N 2  to the carbon monoxide conversion section ( 13, 14 ).

FIELD OF APPLICATION

[0001] The present invention relates to a process for the production ofsynthesis gas for obtaining compounds such as ammonia and methanol.

[0002] More specifically, the invention relates to a process for theproduction of synthesis gas comprising the steps of:

[0003] feeding a first flow comprising hydrocarbons and a first gas flowcomprising steam to a primary reforming section;

[0004] feeding a first gas flow comprising oxygen and possibly nitrogento a secondary reforming section;

[0005] reacting the hydrocarbons and steam first in the primaryreforming section and then—together with oxygen—in the secondaryreforming section, obtaining a first gas phase comprising CO, CO₂, H₂and possibly N₂;

[0006] feeding the first gas phase comprising CO, CO₂, H₂ and possiblyN₂ to a carbon monoxide conversion section.

[0007] Throughout this specification and the appended claims, the term:“hydrocarbons” is used to indicate generically a raw material source ofhydrogen and carbon, such as for example methane, natural gas, naphtha,GPL, (liquefied petroleum gas) or refinery gas and mixtures thereof.

[0008] The invention relates also to a plant for the production ofsynthesis gas for implementing the aforesaid process, as well as to amethod of retrofitting an existing plant for the production of synthesisgas.

[0009] As known, in the field of synthesis gas production, more and morethe need is felt of realising processes which are easy to implement andallow to reach higher and higher production capacities with lowoperating and investment costs and low energy consumption.

PRIOR ART

[0010] In order to satisfy such requirements, synthesis gas productionprocesses, in which a flow comprising hydrocarbons and a gas flowcomprising steam are sent first to a primary reforming section andthen—together with a gas flow comprising oxygen and possibly nitrogen—toa secondary reforming section, have found broad application. A gas phaserich in CO, CO₂, H₂ and possibly N₂ is thereby obtained, which is inturn sent to treatment sections such as for example the carbon monoxideconversion sections at high and low temperature. The treatment sectionscan vary depending on the type of synthesis gas to be produced.

[0011] In order to improve the conversion yield of hydrocarbons, as wellas to reduce the energy consumption, processes for the production ofsynthesis gas are used in the field wherein the conversion reaction inthe secondary reforming section is carried out in the presence of acatalyst.

[0012] The secondary reformers intended for carrying out such processesare generally called autothermal, since they do not require externalheat supply for their operation.

[0013] Although advantageous in some aspects, the above describedprocesses exhibit a series of drawbacks. First of all the fact of beinglittle flexible and not able to adapt themselves effectively tovariations in the operating conditions, in particular when significantincreases in the amount of synthesis gas to be produced are required.

[0014] In fact, the primary and secondary reforming sections,responsible for the conversion of hydrocarbons, are not able to operateconveniently apart from the design capacity.

[0015] Therefore, in order to adapt the synthesis gas producing plantswhich operate according to the above described processes to the capacityincreases required more and more in this field, dramatic interventionsof retrofitting and, last but not least, the replacement of thereforming sections themselves with sections having increased capacityare necessary, with very high investment costs.

[0016] Further on, it is important to notice that the presence of aprimary reforming section requires a supply from outside of high amountsof heat that affects negatively the overall energy consumption necessaryfor implementing such processes.

[0017] Because of these drawbacks, the implementation of synthesis gasproducing processes according to the prior art requires today highinvestments and energy consumption, such to penalise remarkably thecosts of base chemicals such as hydrogen and carbon monoxide, despitethe ever increasing demand for these products.

SUMMARY OF THE INVENTION

[0018] The problem underlying the present invention is to provide aprocess for the production of synthesis gas which is easy to implementand allows to obtain high production capacities with low operating andinvestment costs as well as with low energy consumption.

[0019] The above problem is solved, according to the invention, by aprocess for the production of synthesis gas of the aforesaid type, whichis characterised in that it comprises the steps of:

[0020] feeding a second flow comprising hydrocarbons, a second gas flowcomprising steam and a second gas flow comprising oxygen and possiblynitrogen to an autothermal reforming section provided in parallel withrespect to the primary and secondary reforming sections;

[0021] reacting the hydrocarbons, steam and oxygen in the autothermalreforming section, obtaining a second gas phase comprising CO, CO₂, H₂and possibly N₂;

[0022] feeding the second gas phase comprising CO, CO₂, H₂ and possiblyN₂ to the carbon monoxide conversion section.

[0023] Throughout this specification and the appended claims, the term:“autothermal reforming section” is used to indicate a reforming sectionwherein hydrocarbons, steam and oxygen are reacted, preferably in thepresence of catalyst, without heat being supplied from outside. In theproduction of synthesis gas for ammonia or methanol, sections of thiskind are generally called secondary reforming sections.

[0024] Advantageously, thanks to the step wherein a second flow ofhydrocarbons is reacted in an autothermal reforming section, it ispossible to face easily and effectively even substantial capacityvariations of the plant implementing the process according to theinvention.

[0025] In fact, according to the present invention, the reformingreaction of hydrocarbons is carried out in two stages, provided inparallel, the former comprising a primary reforming section and asecondary reforming section, the latter comprising an autothermalreforming section.

[0026] In this way, it is possible to apportion the desired totalproduction of synthesis gas in the two reforming stages, whose capacitymay be therefore varied from time to time and independently according tothe specific demand, without negatively affecting the remaining process.

[0027] In particular, the load partition in the reforming sectionsarranged in parallel, allows—inter alia—to optimise the energyconsumption, maximising the production of synthesis gas in theautothermal reforming section and at the same time minimising the feedto the primary reformer.

[0028] In other words, the production capacity of synthesis gas beingequal, the present process permits to suitably apportion in tworeforming stages arranged in parallel the hydrocarbons and the steam.Therefore the overall energy consumption is lower than that needed bythe prior art.

[0029] Advantageously, the gas flows comprising CO, CO₂, H₂ and possiblyN₂ obtained respectively in the secondary reforming section and in theautothermal reforming section, are sent to a same carbon monoxideconversion section, exploiting in this way only one equipment line inorder to carry out the subsequent steps of preparation for the synthesisgas.

[0030] A further advantage, resulting from the process according to theinvention, is given by the fact that, having the possibility of feedingseparate flows of hydrocarbons to reforming stages independent from eachother, it is advantageously possible to use for the production ofsynthesis gas hydrocarbons of different nature in the differentreforming stages, thus adapting the process to the existing naturalresources and to whichever requirement may arise.

[0031] In order to obtain a synthesis gas for the production of ammoniawith a high CO₂/H₂ molar ratio, the second gas flow comprising oxygenfed to the autothermal reforming section comprises advantageously oxygenenriched air.

[0032] Throughout this specification and the appended claims, the term:“oxygen enriched air” is used to indicate air with a molar oxygencontent above 21%, for example comprised between 22% and 80%.

[0033] This feature is particularly advantageous for a subsequent ureasynthesis, since it allows the achievement—effectively and cheaply—of aCO₂/NH₃ stoichiometric ratio and therefore to increase the conversionyield of fed carbon into CO₂ and thus urea.

[0034] For the implementation of the above process, the presentinvention provides advantageously a plant for producing synthesis gascomprising:

[0035] a primary reforming section and a secondary reforming sectionarranged in series for obtaining a first gas phase comprising CO, CO₂,H₂ and possibly N₂;

[0036] respective means for feeding a first flow comprising hydrocarbonsand a first gas flow comprising steam to the primary reforming section;

[0037] means for feeding a first gas flow comprising oxygen and possiblynitrogen to the secondary reforming section;

[0038] means for feeding the first gas phase comprising CO, CO₂, H₂ andpossibly N₂ to a carbon monoxide conversion section;

[0039] which is characterised by the fact of comprising:

[0040] an autothermal reforming section for obtaining a second gas phasecomprising CO, CO₂,H₂ and possibly N₂ ;

[0041] respective means for feeding a second flow comprisinghydrocarbons, a second gas flow comprising steam and a second gas flowcomprising oxygen and possibly nitrogen to the autothermal reformingsection;

[0042] means for feeding the second gas phase comprising CO, CO₂, H₂ andpossibly N₂ to the carbon monoxide conversion section.

[0043] According to a further aspect of the invention, it is provided amethod of retrofitting a plant for the production of synthesis gas ofthe type comprising a primary reforming section and a secondaryreforming section arranged in series for obtaining a first gas phasecomprising CO, CO₂, H₂ and possibly N₂, respective means for feeding afirst flow comprising hydrocarbons and a first gas flow comprising steamto the primary reforming section, means for feeding a first gas flowcomprising oxygen and possibly nitrogen to the secondary reformingsection, means for feeding the first gas phase comprising CO, CO₂, H₂and possibly N₂ to a carbon monoxide conversion section, the methodcomprising the steps of:

[0044] providing an autothermal reforming section for obtaining a secondgas phase comprising CO, CO₂, H₂ and possibly N₂;

[0045] providing respective means for feeding a second flow comprisinghydrocarbons, a second gas flow comprising steam and a second gas flowcomprising oxygen and possibly nitrogen to the autothermal reformingsection;

[0046] providing means for feeding the second gas phase comprising CO,CO₂, H₂ and possibly N₂ to the carbon monoxide conversion section.

[0047] Thanks to the aforesaid method of retrofitting it is possibleeasily to increase remarkably the production capacity of an existingplant for the production of synthesis gas, with low operating andinvestment costs and with low energy consumption.

[0048] The characteristics and advantages of the invention will furtherresult from the following description of an embodiment thereof given byway of non limiting example with reference to the attached drawing.

BRIEF DESCRIPTION OF THE FIGURE

[0049]FIG. 1 shows a block diagram of the process for the production ofsynthesis gas according to the invention, in case ammonia and urea arethe wished products.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0050] In FIG. 1, a block diagram is shown that illustrates the processsteps according to the present invention for the production of gaseousreactants, such as H₂, N₂ and CO₂ wherein: H₂ and N₂ are to be used forthe synthesis of ammonia, and CO₂ is used together with the so producedammonia for the synthesis of urea.

[0051] The present process is however particularly indicated for theproduction of gaseous reactants not only for the synthesis of ammoniabut also for the synthesis of methanol, and for various organicreactions which require H₂, CO and possibly N₂ and CO₂.

[0052] With 10 a block diagram is generally indicated which illustratesthe process steps for the production of ammonia and urea, and in whichthe process for the production of synthesis gas according to theinvention is included.

[0053] In diagram 10, blocks 11-18 respectively indicate: a primaryreformer section (block 11), a secondary reformer section (block 12), aCO conversion section, a CO₂ separation section (block 15), apurification section for the synthesis gas (block 16), an ammoniasynthesis section (block 17) and a urea synthesis section (block 18).

[0054] According to the type of synthesis gas to be produced, the COconversion section can be divided in one or more parts. In the exampleof FIG. 1, the CO conversion section comprises a CO conversion sectionat high temperature (block 13) and a CO conversion section at lowtemperature (block 14).

[0055] Blocks 19 and 20 advantageously indicate a (facultative)pre-reforming section (block 19) and an autothermal reforming section(block 20). Blocks 19 and 20 are provided in parallel with respect toblocks 11 and 12.

[0056] The autothermal reforming section (block 20) operates with lowenergy consumption and can include a catalyst bed to facilitate thehydrocarbons reforming reaction.

[0057] Blocks 11-18 are crossed by a flow line 1 that represents a flowhaving a composition which varies with the passage through the differentreaction sections.

[0058] In particular, at the inlet of the primary reforming sectionindicated by block 11, the flow line 1 comprises a first flow comprisinghydrocarbons and a first gas flow comprising steam fed into the flowline 1 by means of the flow line 2.

[0059] Hydrocarbons entering the primary reforming section (block 11)are preferably of gaseous type as for example natural gas.

[0060] The flow line 3 indicates a first gas flow comprising oxygen fedinto the secondary reforming section (block 12).

[0061] Passing through the primary reforming section and the secondaryreforming section (blocks 11 and 12) arranged in series, hydrocarbonsand steam contained in the feed flow 1 —together with oxygen comprisedin the flow 3—react, thus obtaining a first gas phase comprising CO, CO₂and H₂.

[0062] The gas phase leaving the secondary reforming section through theflow line 1, will further comprise a suitable amount of nitrogen (N₂)necessary for the following synthesis of ammonia in block 17.

[0063] To this end, the gas flow comprising oxygen fed by the flow line3 into block 12, comprises also nitrogen. Preferably, the flow line 3represents an air flow.

[0064] According to which kind of final synthesis is desired, the flowline 3 can be used for feeding substances of different nature. Forexample, in the case of methanol synthesis, the flow line 3 feeds onlyappropriate amount of oxygen to the secondary reforming section.

[0065] The carbon monoxide comprised in the gas phase leaving block 12is thus converted into carbon dioxide through the conversion sections athigh and low temperature (blocks 13 and 14), subsequently separated inthe CO₂ separation section (block 15) and finally fed as reactant forthe urea synthesis through flow line 4 into block 18.

[0066] From block 15, the gas phase substantially free of CO and CO₂,passes through the synthesis gas purification section (block 16) and isthen fed—in the form of a gas flow comprising essentially hydrogen andnitrogen—to the ammonia synthesis section indicated with block 17.

[0067] The produced ammonia leaving block 17, is then sent—alwaysthrough the flow line 1—to the urea synthesis section (block 18), whereit reacts with the carbon dioxide coming from the CO₂ separation section(block 15).

[0068] Therefore, the flow leaving block 18 (flow line 1) mainlycomprises urea.

[0069] Advantageously, a second flow line indicated with 5 in FIG. 1crosses blocks 19 and 20 of diagram 10.

[0070] At the inlet to the pre-reforming section (block 19), the flowline 5 comprises a second flow comprising hydrocarbons and a second gasflow comprising steam fed into the flow line 5 by means of the flow line6.

[0071] Hydrocarbons fed to such section may be of the same kind of thosefed to the reforming sections indicated with blocks 11 and 12, or ofdifferent kind such as for example naphtha.

[0072] In particular, thanks to the presence of the pre-reformingsection (block 19), it is possible to use advantageously practically anykind of hydrocarbons for the reforming reaction, and to obtain at thesame time a reduction of the energy and feed steam consumption.

[0073] In this way it is possible to adapt the process for theproduction of synthesis gas to whichever condition and type ofhydrocarbon mixtures available.

[0074] Block 19 is anyway to be considered as optional and notnecessary, in particular when gaseous hydrocarbons such as natural gasare used for the reforming reaction.

[0075] In this respect, it shall be clarified that it is absolutely notnecessary to feed all the second flow comprising hydrocarbons and allthe second flow comprising steam to the pre-reforming section (block19). In fact, in some instances, depending upon the working conditionsand upon the kind of hydrocarbons available, it can be more advantageousto send only a part of such flows (for example between 20% and 80%) toblock 19, and the remaining part directly to the autothermal reformingsection (block 20).

[0076] Further on, a second gas flow comprising oxygen and in this casealso nitrogen, for example air, is sent to the autothermal reformingsection (block 20) by means of the flow line 7, analogously to whatalready described with respect to flow line 3.

[0077] Passing through the pre-reforming section and the autothermalreforming section (blocks 19 and 20), the hydrocarbons and the steamcontained in feed flow 5 react, obtaining a second gas phase comprisingCO, CO₂, H₂ and N₂ which is combined with the first gas phase (flow line1) immediately upstream of the CO conversion section and together withthe it passes through the remaining blocks of the diagram 10, as abovedescribed.

[0078] In the example shown in FIG. 1, the flow line 5 enters into theflow line 1 upstream of the high temperature conversion sectionindicated by block 13. Anyway, the possibility is not excluded, even ifnot represented, of sending at least a portion of the second gas phasecoming from the autothermal reforming section (block 20) to a locationupstream of the CO conversion section at low temperature, between blocks13 and 14.

[0079] Particularly satisfactory results have been obtained by feedingoxygen enriched air to block 20 through the flow line 7.

[0080] In doing so, the amount of CO₂ comprised in the second gas phaseand therefore which can be fed to the urea synthesis section (block 18)is advantageously increased, thereby improving the conversion yield.Controlling the concentration and the feed rate of the flow comprisingoxygen enriched air fed to the autothermal reforming section, it ispossible to obtain CO₂ in a sufficient amount to convert all the ammoniaproduced into urea, and this independently from the type of hydrocarbonsfed to blocks 1 and 5.

[0081] Further on, the use of oxygen enriched air in the present processallows to reduce the amount of inert gases sent to the ammonia synthesissection (block 17) advantageously increasing the conversion yield insuch section.

[0082] According to an alternative embodiment of the present invention,it is foreseen to divert a part of the flow comprising feed hydrocarbonsfrom the flow line 1 to the flow line 5 to be sent to the autothermalreforming section (block 20), as indicated by the flow line 8represented with a dashed line.

[0083] In this way, whenever the maximum operating capacity of the plantimplementing the present process is not required, it is possible tofurther reduce the overall energy consumption, because the load to theautothermal reforming section (block 20) can be maximised and theexternal energy supply to the primary reforming section (block 11) canbe reduced.

[0084] Preferably, the portion of the first flow of hydrocarbonsdiverted to the flow line 5 is comprised between 5% and 60% on total.

[0085] Alternatively, according to a not represented embodiment of thepresent invention, the flow line 8 departs from the flow line 1 in alocation downstream to the inlet into line 1 of flow line 2. In thiscase, along with a portion of the first flow comprising hydrocarbonsalso a portion of the first gas flow comprising steam is fed into flowline 5.

[0086] Generally speaking, the very high flexibility of the processaccording to the invention allows to reduce, depending from the flowrates and the amount of synthesis gas to be produced, the load to theprimary reforming section with a corresponding advantage in terms ofenergy consumption.

[0087] In this respect, particularly satisfying results have beenobtained minimising the amount of hydrocarbons fed to the primaryreforming section and at the same time maximising the amount ofhydrocarbons to be sent to the autothermal reforming section.

[0088] The operating conditions of the sections indicated by blocks11-20, as well as the nature of the chemical reactions occurringtherein, are conventional and therefore will not be further describedbeing known to the man skilled in the art.

[0089] According to the process for the production of synthesis gas ofthe present invention, a first flow comprising hydrocarbons and a firstgas flow comprising steam are fed (flow line 1 and 2) to a primaryreforming section (block 11), while a first gas flow comprising oxygenand possibly nitrogen (flow line 3) is fed to a secondary reformingsection (block 12). The hydrocarbons and the steam are reacted in theprimary reforming section and then—together with oxygen—in the secondaryreforming section, obtaining a first gas phase comprising CO, CO₂, H₂and possibly N₂. The so obtained gas phase is then fed to a carbonmonoxide conversion section.

[0090] Advantageously, according to the further steps of the presentprocess, a second flow comprising hydrocarbons, a second gas flowcomprising steam and a second gas flow comprising oxygen and possiblynitrogen (flow lines 5-7) are fed to an autothermal reforming section(block 20) arranged in parallel with respect to the primary andsecondary reforming sections. The hydrocarbons, the steam and the oxygenare reacted in the autothermal reforming section obtaining a second gasphase comprising CO, CO₂, H₂ and possibly N₂ that is in turn sent (flowline 5) to the carbon monoxide conversion section.

[0091] According to an alternative embodiment, the process according tothe present invention further comprises the step of subjecting at leasta part of the second flow comprising hydrocarbons and of the second gasflow comprising steam to a pre-reforming treatment (block 19) beforebeing fed to the autothermal reforming section.

[0092] According to a further alternative embodiment, the presentprocess foresees furthermore the step of feeding (flow line 8) a portionof the first flow comprising hydrocarbons to the autothermal reformingsection.

[0093] The plant for producing synthesis gas according to the presentinvention comprises the sections indicated by blocks 11-20 of FIG. 1.

[0094] Suitable feeding and connecting means are foreseen at the inletand between the single sections that build up the aforesaid plant,respectively. These means are of known type, such as for example ducts,pipes or alike, schematically represented by the flow lines 1-8 of FIG.1. Conventional heat exchangers—not represented in FIG. 1—may also beprovided in the plant.

[0095] A particularly important aspect of the present invention isrepresented by the retrofitting of pre-existing plants for theproduction of synthesis gas.

[0096] In this respect, the invention provides for a method ofretrofitting a plant for the production of synthesis gas of the typecomprising a primary reforming section, a secondary reforming sectionand a carbon monoxide conversion section (blocks 11-14) connected inseries, method which advantageously comprises the steps of providing anautothermal reforming section (block 20) in parallel to the existingreforming sections and suitable means for feeding into the autothermalreforming section a second flow comprising hydrocarbons, a second gasflow comprising steam and a second gas flow comprising oxygen andpossibly nitrogen, respectively, as well as connecting means between theautothermal reforming section and the carbon monoxide conversion section(flow lines 5-7).

[0097] Thanks to the present method of retrofitting, it is possible toincrease remarkably the production capacity of an existing plant, forexample from 20 to 70%, without overloading the reforming sections andabove all maintaining low energy consumption and operating costs if noteven reducing them.

[0098] Further on, once retrofitted, the plant gains a higherflexibility, being in the condition of suitably operating with whichevertype of hydrocarbon and working condition.

[0099] In particular, it is possible to apportion the loads between thereforming stages arranged in parallel, in such a way to minimise theprimary reforming conversion and accordingly optimise the energyconsumption.

[0100] It is important to notice that—advantageously—the method ofretrofitting according to the invention does not require to enhance orreplace the existing reforming sections.

[0101] In addition thereto, also the downstream sections for thetreatment of the produced synthesis gas are not subjected to particularoverloads, requiring—if the case—only marginal and inexpensiveinterventions. It shall be noticed that, a possible replacement orsubstantial modification of such sections implies however a much lowercost than the modification of one or even two reforming sections.

[0102] According to a preferred embodiment of the present method ofretrofitting, the second gas flow comprising oxygen (flow line 7) fed tothe autothermal reforming section (block 20) comprises oxygen enrichedair. In doing so, it is advantageously possible to increase the amountof CO₂ produced, for example until the CO₂/NH₃ stoichiometric ratio forurea synthesis is achieved, independently from the type of hydrocarbonbeing fed.

[0103] In order to further reduce the energy consumption, the method ofretrofitting according to the present invention advantageously foreseesthe step of providing means for feeding a portion of the first flowcomprising hydrocarbons to the autothermal reforming section (flow line8).

[0104] Alternatively, together with the portion of flow comprisinghydrocarbons, also a portion of the gas flow comprising steam is sent tothe flow line 5. In this case, the hydrocarbons and the steam to be sentto the autothermal reforming section are preferably taken out alreadysuitably mixed and pre-heated from the flow line 1. In doing so, it ispossible to reduce if not even to eliminate the respective apparatusesfor mixing and pre-heating the reactants to be sent to the autothermalreforming section, with ensuing energy and investment costs savings.

[0105] Finally, according to a further embodiment of the method ofretrofitting according to the present invention, the steps of providinga pre-reforming section (block 19), and of providing feeding means of atleast a part of the second flow comprising hydrocarbons and of thesecond gas flow comprising steam to such pre-reforming section andconnecting means between the pre-reforming section and the autothermalreforming section (flow line 5), are provided.

[0106] In this way, it is possible to use essentially any kind ofhydrocarbons as source of carbon and hydrogen to be sent to theautothermal reforming section, without affecting the operation thereof,but, to the opposite, allowing a reduction in the amount of steam to besent to such section, with ensuing savings in terms of energyconsumption and operating costs.

EXAMPLE

[0107] In the following example, the advantages resulting from themethod of retrofitting according to the present invention are displayed.

[0108] In particular, the energy consumption relative to a capacityincrease equal to 50% of an existing plant for the production ofsynthesis gas for obtaining ammonia is discussed.

[0109] The results of the instant example have been obtained by means ofcommercially available calculation algorithms.

[0110] The existing plant is of the type shown and described withreference to FIG. 1, blocks 11-17, and was designed to operated at anaverage production capacity of 1000 MTD of ammonia. The overall energyconsumption is normally of 8300 kcal/MT of ammonia.

[0111] Natural gas is used as a source of hydrocarbons and the gas flowcomprising oxygen fed to the secondary reforming section consists ofair.

[0112] The primary and secondary reforming sections of the existingplant were not designed for facing a capacity increase equal to 50% buton the contrary, can, at most, stand production peaks that do notoverride the average value by more than 10-15%.

[0113] According to the method of retrofitting of the present invention,the capacity increase of such plant by 50%, for an overall production of1500 MTD of ammonia, is obtained adding in parallel a suitablydimensioned autothermal reforming section fed with air, steam, naphthaand a portion of the natural gas flow coming from the existing plant(see FIG. 1, reference signs 5-8, 20).

[0114] The load is advantageously split in such a way to carry out 60%of the overall production in the existing reforming step (900 MTD) andthe remaining 40% in the autothermal reforming section (600 MTD).

[0115] Thanks to the present method of retrofitting, it has beensurprisingly noticed that—notwithstanding a capacity increase equal to50%—the overall energy consumption has even decreased of a 2-3% withrespect to the existing plant and corresponds to about 8100 kcal/MT ofammonia. Compared with a retrofitting carried out according to the priorart, which foresees the replacement of the existing primary andsecondary reforming sections with new reforming sections having acapacity increased by 50%, the method of retrofitting according to thepresent invention is extremely advantageous both for the lower energyconsumption and—especially—for the lower investment costs.

[0116] Finally, it's worth stressing that in order to implement thepresent method, the same does not require long shutdown times of theexisting plant, in view of the fact that the autothermal reformingsection is erected in parallel to the existing sections. In this way,the existing plant may operate until the connection between theadditional section and the carbon monoxide conversion section isrealised.

[0117] On the contrary, according to the prior art the plant must beshut down for a longer period in order to permit the retrofitting or thereplacement of the reforming sections, with ensuing relevant productionlosses.

[0118] From what above disclosed, the numerous advantages achieved bythe present invention clearly arise; in particular it is possible toobtain an extremely flexible process for the production of synthesisgas, easy to implement and which allows to achieve high productioncapacities with low operating and investment costs and low energyconsumption.

1. Process for the production of synthesis gas for obtaining compounds such as ammonia or methanol, comprising the steps of: feeding a first flow comprising hydrocarbons and a first gas flow comprising steam to a primary reforming section; feeding a first gas flow comprising oxygen and possibly nitrogen to a secondary reforming section; reacting said hydrocarbons and said steam first in the primary reforming section and then—together with said oxygen—in the secondary reforming section, obtaining a first gas phase comprising CO, CO₂, H₂ and possibly N₂; feeding said first gas phase comprising CO, CO₂, H₂ and possibly N₂ to a carbon monoxide conversion section; characterised in that it further comprises the steps of: feeding a second flow comprising hydrocarbons, a second gas flow comprising steam and a second gas flow comprising oxygen and possibly nitrogen to an autothermal reforming section arranged in parallel with respect to said primary and secondary reforming sections; reacting said hydrocarbons, said steam and said oxygen in said autothermal reforming section, obtaining a second gas phase comprising CO, CO₂, H₂ and possibly N₂; feeding said second gas phase comprising CO, CO₂, H₂ and possibly N₂ to said carbon monoxide conversion section.
 2. Process according to claim 1 , characterised in that said second gas flow comprising oxygen comprises oxygen enriched air.
 3. Process according to claim 1 , characterised in that it further comprises the step of subjecting at least a part of said second gas flow comprising hydrocarbons and said second gas flow comprising steam to a pre-reforming treatment before being fed to said autothermal reforming section.
 4. Process according to claim 1 , characterised in that it further comprises the step of feeding a portion of said first flow comprising hydrocarbons to said autothermal reforming section.
 5. Plant for producing synthesis gas for obtaining compounds such as ammonia or methanol, comprising: a primary reforming section (11) and secondary reforming section (12) arranged in series for obtaining a first gas phase comprising CO, CO₂, H₂ and possibly N₂; respective means (1, 2) for feeding a first flow comprising hydrocarbons and a first gas flow comprising steam to said primary reforming section (11); means (3) for feeding a first gas flow comprising oxygen and possibly nitrogen to said secondary reforming section (12); means (1) for feeding said first gas phase comprising CO, CO₂, H₂ and possibly N₂ to a carbon monoxide conversion section (13, 14); characterised in that it comprises: an autothermal reforming section (20) for obtaining a second gas phase comprising CO, CO₂, H₂ and possibly N₂ ; respective means (5-7) for feeding a second flow comprising hydrocarbons, a second gas flow comprising steam and a second gas flow comprising oxygen and possibly nitrogen to said autothermal reforming section (20); means (5) for feeding said second gas phase comprising Co, CO₂, H₂ and possibly N₂ to said carbon monoxide conversion section (13, 14).
 6. Plant according to the claim 5 , characterised in that it further comprises: a pre-reforming section (19); means (5) for feeding at least a part of said second gas flow comprising hydrocarbons and said second gas flow comprising steam to said pre-reforming section (19); means (5) for connecting said pre-reforming section (19) with said autothermal reforming section (20).
 7. Plant according to claim 5 , characterised in that it further comprises means (8) for feeding a portion of said first flow comprising hydrocarbons to said autothermal reforming section (20).
 8. Method of retrofitting a plant for the production of synthesis gas for obtaining compounds such as ammonia or methanol, of the type comprising a primary reforming section (11) and a secondary reforming section (12) provided in series for obtaining a first gas phase comprising CO, CO₂, H₂ and possibly N₂, respective means (1, 2) for feeding a first flow comprising hydrocarbons and a first gas flow comprising steam to said primary reforming section (11), means (3) for feeding a first gas flow comprising oxygen and possibly nitrogen to said secondary reforming section (12), means (1) for feeding said first gas phase comprising CO, CO₂, H₂ and possibly N₂ to a carbon monoxide conversion section (13, 14), said method comprising the steps of: providing an autothermal reforming section (20) for obtaining a second gas phase comprising CO, CO₂, H₂ and possibly N₂; providing respective means (5-7) for feeding a second flow comprising hydrocarbons, a second gas flow comprising steam and a second gas flow comprising oxygen and possibly nitrogen to said autothermal reforming section (20); providing means (5) for feeding said second gas phase comprising CO, CO₂, H₂ and possibly N₂ to said carbon monoxide conversion section (13, 14).
 9. Method according to claim 8 , characterised in that it further comprises the steps of: providing a pre-reforming section (19); providing means (5) for feeding at least a part of said second flow comprising hydrocarbons and said second gas flow comprising steam to said pre-reforming section (19); providing means (5) for connecting said pre-reforming section (19) with said autothermal reforming section (20).
 10. Method according to claim 8 , characterised in that it further comprises the steps of: providing means (8) for feeding a portion of said first flow comprising hydrocarbons, and possibly a portion of said first gas flow comprising steam, to said autothermal reforming section (20). 